Cushing's disease is caused by the pituitary hypersecretion of ACTH and occurs predominantly in women. Patients are cured by surgical removal of an ACTH-producing adenoma, suggesting evolution and expansion of a genetically aberrant cell. However, hypothalamic dysfunction and excessive stimulation of anterior pituitary corticotrophs by one or more neurotransmitter substances may also lead to the development of corticotropic adenomas. To study the clonal composition of ACTH-producing pituitary adenomas, we used restriction fragment length polymorphism (RFLP) of two X-chromosome linked genes, hypoxanthine phosphoribosyl transferase (HPRT) and phosphoglycerate kinase (PGK), in conjunction with their methylation patters. Analysis of 27 tumors demonstrated a monoclonal pattern in six of these tumors, whereas a polyclonal pattern was observed in three tumors including a pituitary adenoma from a patient with the Nelson's syndrome. Amplification by the polymerase chain reaction, (PCR), of the mutational host spots in ras genes followed by allele specific oligonucleotide (ASO), hybridization showed mutation in one of the 27 tumors. We also searched for mutations in the promoter region of the POMC gene by PCR amplification and sequencing. This region was found to be perfectly normal in 11 tumors. Allelotyping of the pituitary tumors is being carried out by using restriction fragment length polymorphism (RFLP) analysis. Initial studies showed loss of heterozygosity of genes on chromosome 17 in one of the 4 Nelson's tumors.